Methods to optimize conformal dynamic arcs in radiation therapy treatment planning.

Conformal Dynamic Arcs (CDA) can provide a useful alternative in certain clinical situations which require a higher level of 3-dimensional (3D) conformation than shaped static fields but for which modulated fields (fixed or arc) are inappropriate. Due to lack of modulation, the quality of the dose distribution produced by a CDA is highly dependent on the specific patient geometry. The ideal geometry for a CDA, in terms of achievable conformation and uniformity, is a spherical target perfectly centered in a cylindrical medium or body and utilizing a full 360 degree of rotation. This manuscript will provide or review several methods a treatment planner may employ to improve dose distributions produced by CDA as the patient and/or target geometry or degrees of arc rotation vary from the ideal. These include: 1. Weighting arc segments to improve homogeneity. 2. Defining asymmetric margins for fitting leaves. 3. Hybrid static/CDA compensating for CDA with limited degrees of rotations. 4. Improving conformation in irregular target via use of pseudo-PTV. 5. Creating concave features via the use of avoidance structures. 6. Editing CDA multi-leaf collimator (MLC) to remove irregular hot spots.

Consideration of dose limits for organs at risk of thoracic radiotherapy: atlas for lung, proximal bronchial tree, esophagus, spinal cord, ribs, and brachial plexus.

PURPOSE: To review the dose limits and standardize the three-dimenional (3D) radiographic definition for the organs at risk (OARs) for thoracic radiotherapy (RT), including the lung, proximal bronchial tree, esophagus, spinal cord, ribs, and brachial plexus. METHODS AND MATERIALS: The present study was performed by representatives from the Radiation Therapy Oncology Group, European Organization for Research and Treatment of Cancer, and Soutwestern Oncology Group lung cancer committees. The dosimetric constraints of major multicenter trials of 3D-conformal RT and stereotactic body RT were reviewed and the challenges of 3D delineation of these OARs described. Using knowledge of the human anatomy and 3D radiographic correlation, draft atlases were generated by a radiation oncologist, medical physicist, dosimetrist, and radiologist from the United States and reviewed by a radiation oncologist and medical physicist from Europe. The atlases were then critically reviewed, discussed, and edited by another 10 radiation oncologists. RESULTS: Three-dimensional descriptions of the lung, proximal bronchial tree, esophagus, spinal cord, ribs, and brachial plexus are presented. Two computed tomography atlases were developed: one for the middle and lower thoracic OARs (except for the heart) and one focusing on the brachial plexus for a patient positioned supine with their arms up for thoracic RT. The dosimetric limits of the key OARs are discussed. CONCLUSIONS: We believe these atlases will allow us to define OARs with less variation and generate dosimetric data in a more consistent manner. This could help us study the effect of radiation on these OARs and guide high-quality clinical trials and individualized practice in 3D-conformal RT and stereotactic body RT.

Vessel-sparing prostate radiotherapy: dose limitation to critical erectile vascular structures (internal pudendal artery and corpus cavernosum) defined by MRI.

PURPOSE: Most evidence suggests that impotence after prostate radiation therapy has a vascular etiology. The corpus cavernosum (CC) and the internal pudendal artery (IPA) are the critical vascular structures related to erectile function. This study suggests that it is feasible to markedly decrease radiation dose to the CC and the IPA and directly determine the impact of dose limitation on potency. METHODS AND MATERIALS: Twenty-five patients (10 external beam, 15 brachytherapy) underwent MRI/CT-based treatment planning for prostate cancer. In addition, 10 patients entered on the vessel-sparing protocol underwent a time-of-flight MRI angiography sequence to define the IPA. The distance from the MRI-defined prostate apex to the penile bulb (PB), CC, and IPA was measured and compared to the distance from the CT-defined apex. Doses (D5 and D50) to the PB, CC, and IPA were determined for an 80 Gy external beam course. In 5 patients, CT plans were generated and compared to MRI-based plans. RESULTS: The combination of coronal, sagittal, and axial MRI data sets allowed superior definition of the prostate apex and its relationship to critical vascular structures. The apex to PB distance averaged 1.45 cm (0.36 standard deviation) with a range of 0.7 cm to 2.1 cm. Peak dose (D5) to the proximal CC in the MRI-planned 80 Gy course was 26 (9) Gy (0.36 of CT-planned dose), and peak dose to the IPA was 39 (13) Gy (0.61 of CT-planned dose). CONCLUSION: The distance between the prostate apex and critical vascular structures is highly variable. Current empiric rules for CT contouring (apex 1.5 cm above PB) overestimate or underestimate the distance between the prostate apex and critical vascular structures. When defined by MRI T2 and MRI angiogram with CT registration, limitation of dose to critical erectile structures is possible, with a more significant gain than has been previously reported using dose limitation by commonly applied intensity modulated radiation therapy studies based on CT imaging. These techniques make "vessel-sparing" prostate radiotherapy feasible.

The use of mutual information in registration of CT and MRI datasets post permanent implant.

PURPOSE: To determine the feasibility of registration of MRI and CT datasets post permanent prostate implant by the use of mutual information. METHODS AND MATERIALS: Five patients who underwent permanent (125)I implant for prostate carcinoma were studied. Two weeks postimplant an axial CT, T2-weighted-axial, sagittal and coronal MRI, and T1-fat-saturation MRI scans were obtained. Registrations of MRI to CT and MRI to MRI datasets were performed by mutual information, an automated process of data registration matching all information in specified dataset regions of interest. Registration quality was evaluated by visual inspection, agreement with seed- to-seed registration, and histogram analysis. RESULTS: Rapid registration (<30 minutes) of CT and MRI datasets can be accomplished through the use of mutual information. All methods of registration evaluation confirmed excellent registration quality. Although D90 and V100 for the prostate were comparable between MRI- and CT-based dosimetry, dose to critical structures/microenvironments (anterior base, posterior base, bladder outlet, lower sphincter, bulbar urethra) defined on MRI varied widely. CONCLUSIONS: Efficient and accurate registration of MRI and CT datasets following prostate implant is possible, and improves the accuracy of postimplant dosimetry by superior definition of the prostate. Definition of critical microenvironments and adjacent structures will improve dose and toxicity correlation and ultimately improve planning strategies.

Recurrences near base of skull after IMRT for head-and-neck cancer: implications for target delineation in high neck and for parotid gland sparing.

PURPOSE: Locoregional (LR) failures near the base of the skull, and their relationships to the targets in the high neck, were examined in a series of patients who underwent intensity-modulated radiotherapy (IMRT) for head-and-neck cancer. METHODS AND MATERIALS: Between 1994 and 2002, 133 patients with non-nasopharyngeal head-and-neck squamous cell carcinoma completed a course of curative, parotid-sparing RT. Treatment was delivered until 1996 with conformal three-dimensional techniques and thereafter with multisegmental static IMRT. Of the 133 patients, 80 had oropharyngeal, 27 oral cavity, 12 hypopharyngeal, 11 laryngeal, and 3 unknown primary cancer. The AJCC stage was I in 1, II in 6, III in 26, IVA in 83, and IVB in 12; cancer was recurrent in 5. Sixty patients received primary and 73 postoperative IMRT. 86% of patients (n = 115) had ipsilateral neck metastasis. In all patients, the contralateral neck was clinically node negative but was judged to be at high risk of subclinical disease. Delineation of the superior-most extent of the nodal targets was consistent and corresponded with Rouviere's observations. In the contralateral clinically node-negative neck, the uppermost Level II nodal target was the subdigastric (SD) nodes. To ensure coverage, the uppermost clinical target volume was delineated at the axial CT image in which the posterior belly of the digastric muscle crossed the jugular vein. In the ipsilateral neck, which was node positive in most patients, the uppermost Level II clinical target volume was delineated through the base of the skull. The uppermost retropharyngeal (RP) nodal target was delineated at the level of the top of the C1 vertebral body, accommodating Rouviere's description of the location of the lateral RP nodes. The dose prescription was 70 Gy for the primary planning target volumes (PTVs); 64 Gy and 60 Gy for PTV of the postoperative beds with and without extracapsular extension, respectively; and 50-54 Gy for PTVs of nonoperated subclinical disease, at 1.8-2.0 Gy fractions. In-field or marginal recurrences were defined as those occurring when >95% or 20-95% of the recurrence volume, respectively, had received >95% of the prescribed dose. RESULTS: At a median follow-up of 32 months (range, 6-107 months), 21 patients (16%) had locoregional recurrence. Seventeen recurrences were in-field and four were marginal. The most prevalent nodal recurrence was in Level II bilaterally, where all failures were in-field. In-field failures were observed in the ipsilateral high neck cranial to the SD nodes (all in initially node-positive neck). No recurrences occurred in the contralateral high neck, cranial to the SD nodes, which was not included in the targets. The 95% confidence interval for the risk of recurrence in that region was 0-2.7%. Three RP nodal failures were observed; two were marginal, occurring ipsilateral and contralateral to the primary tumors, and centered cranial to the top of C1. After these recurrences, the RP nodal clinical target volumes were delineated bilaterally through the base of skull, without subsequent RP recurrence. These target delineation guidelines allowed the achievement of a mean contralateral parotid dose of < or =26 Gy (found previously to preserve salivary output significantly) in 82% of the patients. The 3-year actuarial LR recurrence-free survival rate of primary and postoperative IMRT patients was similar (81% and 84%, respectively). Oropharyngeal cancer patients had the greatest LR recurrence-free survival rate (94%, p <0.001). No statistically significant differences were found in the dose delivered to the PTVs or the in-field recurrence volume between patients who had or did not have LR failure. CONCLUSION: These results suggest that when the contralateral node-negative side of the neck has a high risk of subclinical metastasis, it is adequate to include the SD nodes as the cranial-most Level II nodal target in non-nasopharyngeal head-and-neck cancer. In the node-positive side of the neck, this nodal level should be delineated more cranially. The RP nodal targets should be delineated more cranially. The RP nodal targets should be delineated bilaterally and should extend to the base of the skull, rather than to the top of C1. These guidelines allowed substantial sparing of the contralateral parotid gland. The results of this series validate a consensus for target delineation adopted recently by cooperative radiotherapy groups.

Salivary gland sparing and improved target irradiation by conformal and intensity modulated irradiation of head and neck cancer.

The goals of this study were to facilitate sparing of the major salivary glands while adequately treating tumor targets in patients requiring comprehensive bilateral neck irradiation (RT), and to assess the potential for improved xerostomia. Since 1994 techniques of target irradiation and locoregional tumor control with conformal and intensity modulated radiation therapy (IMRT) have been developed. In patients treated with these modalities, the salivary flow rates before and periodically after RT have been measured selectively from each major salivary gland and the residual flows correlated with glands' dose volume histograms (DVHs). In addition, subjective xerostomia questionnaires have been developed and validated. The pattern of locoregional recurrence has been examined from computed tomography (CT) scans at the time of recurrence, transferring the recurrence volumes to the planning CT scans, and regenerating the dose distributions at the recurrence sites. Treatment plans for target coverage and dose homogeneity using static, multisegmental IMRT were found to be significantly better than standard RT plans. In addition, significant parotid gland sparing was achieved in the conformal plans. The relationships among dose, irradiated volume, and the residual saliva flow rates from the parotid glands were characterized by dose and volume thresholds. A mean radiation dose of 26 Gy was found to be the threshold for preserved stimulated saliva flow. Xerostomia questionnaire scores suggested that xerostomia was significantly reduced in patients irradiated with bilateral neck, parotid-sparing RT, compared to patients with similar tumors treated with standard RT. Examination of locoregional tumor recurrence patterns revealed that the large majority of recurrences occurred inside targets, in areas that had been judged to be at high risk and that had received RT doses according to the perceived risk. Tangible gains in salivary gland sparing and target coverage are being achieved, and an improvement in some measures of quality of life is suggested by our findings. Additional reduction of xerostomia may be achieved by further sparing of the salivary glands and the non-involved oral cavity. A mean parotid gland dose of < or = 26 Gy should be a planning objective if significant parotid function preservation is desired. The pattern of recurrence suggests that careful escalation of the dose to areas judged to be at highest risk may improve tumor control.

A fluence convolution method to account for respiratory motion in three-dimensional dose calculations of the liver: a Monte Carlo study.

We describe the implementation of a fluence convolution method to account for the influence of superior-inferior (SI) respiratory induced motion on a Monte Carlo-based dose calculation of a tumor located in the liver. This method involves convolving the static fluence map with a function describing the SI motion of the liver-the motion function has been previously derived from measurements of diaphragm movement observed under fluoroscopy. Significant differences are noted between fluence-convolved and static dose distributions in an example clinical treatment plan; hot and cold spots (on the order of 25%) are observed in the fluence-convolved plan at the superior and inferior borders of the liver, respectively. This study illustrates that the fluence convolution method can be incorporated into Monte Carlo dose calculation algorithms to account for some of the effects of patient breathing during radiotherapy treatment planning, thus leading to more accurate dose calculations.